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Abstract
NSTX is a spherical tokamak (ST) that operates with ne up to 1020 m-3 and BT less than 0.6 T, cutting off low harmonic electron cyclotron (EC) waves widely used for electron temperature measurements and EC heating and current drive in conventional aspect ratio tokamaks. The electron Bernstein wave (EBW) can propagate in ST plasmas and is readily absorbed and emitted at EC harmonics. Additionally, EBWs do not experience a density dependent cutoff. As such, EBWs may enable local electron temperature measurements and provide local electron heating and current drive. EBWs cannot propagate in vacuum but can couple to electromagnetic waves, so for these applications efficient coupling between the EBWs and electromagnetic waves outside the plasma is needed.
In this thesis, EBW emission via the oblique double mode conversion process to the X- and O-modes (B-X-O) is measured with two remotely steered antennas located outside of the vacuum vessel. These emission measurements have been used to determine the EBW transmission efficiency for a wide range of plasma conditions. The antennas collect fundamental (8-18 GHz), second and third (18-40 GHz) harmonic emission. The remote steering capability allowed detailed measurements (as a function of toroidal and poloidal pointing angle) of the B-X-O transmission window. Peak L-mode B-X-O transmission efficiencies of 90% and 35% for fundamental and second harmonic emission, respectively, were measured. The measured and theoretical optimal pointing angles agreed within 5° of the simulated values. Evidence of strong EBW collisional damping near the fundamental and second harmonic B-X-O conversion region was observed in H-mode discharges, reducing the B-X-O transmission efficiency to nearly 0% in some cases. Edge conditioning, via Li evaporation, successfully reduced this EBW damping and increased transmission efficiencies to 50-60%, agreeing with EBW emission (EBE) simulations. These results provide experimental evidence supporting B-X-O mode conversion theory and provide verification of an EBE simulation code developed at the Czech Institute of Plasma Physics. Additionally, the results presented in this thesis provide the first measurements demonstrating that EBW collisional damping can be successfully reduced with edge conditioning. Except for power dependent effects, the physics of B-X-O emission and O-X-B injection are reciprocal, thus the B-X-O mode conversion measurements support the feasibility of EBW based heating and current drive experiments for future ST devices such as an ST-based Component Test Facility.
Te(R) reconstructions of ST H-mode plasma via EBE measurements are also presented in this thesis and have been quite difficult. The B-X-O process is a complicated process, involving double mode conversion of the emission before it is detected, and an antenna view oblique to the magnetic field near the plasma edge, so that extensive numerical modeling is needed to determine the emission location and reconstruct the Te (R) profile. In contrast to stellarators or conventional aspect ratio tokamaks, reconstruction of the magnetic equilibrium in an ST is strongly affected by time varying internal currents that generate poloidal fields that can be comparable to the toroidal field. Good agreement with the edge Thomson scattering and EBE Te(R) measurements have been observed. Inside a major radius of 140 cm, EBE Te(R) measurements are within 40% of Thomson scattering measurements.
